Method of incorporating resin in molded pulp products



Nov. 17, 1964 D. A. BRAFFORD METHOD OF INCORPORATING RESIN IN MOLDEDPULP PRODUCTS Filed July 17, 1962 oncza A. lfafof'd B INVENTOR.

Y M da cy laoRNEi/s United States Patent O 3,157,566 lviE'HD 9FEQCRPRATNG RESBN iN MGLDED PULP PRDUCT S Dond A. Brantford, Beloit,Wis., assigner to leloitCorporation, Beloit, Wis., a corporation ofWisconsin Filed luiy 17, 1962, Ser. No. 210,547 7 Claims. (Ci. 162-146)This invention relates to an improved method of forming molded pulparticles, and more particularly, to an improved method of eecting vresinimpregnation 'of articles formed by the deposition of brous pulp onto aporous forming surface.

Although the instant invention may have a number of uses, it isparticularly adapted to uses which relate generally to the papertreating art, and the instant invention will be described primarily inconnection therewith. In general, in the paper making or forming art,the fibrous pulp particles in a dilute aqueous slurry (commonly referredto as stoc are formed into the desired shape initially by forcing theslurry through a porous forming surface, which might be a forming wireor a porous mold form of one shape or another, so as to deposit a layerof such iibers on the forming surface. The instant invention may be usedin the formation of a sheet on a forming Wire or a molded pulp articleon a porous mold surface; and the instant invention is particularlyadapted for use in the formation of relatively thick Walled pulparticles of generally cylindrical or shell-like shape having arelatively large cylinderV length-to-diameter ratio.

Heretofore, it was known that the wet strength and other properties ofpaper might be increased by the incorporation in the dilute aqueousstock of certain small amounts of water-soluble resins, but thisprocedure involved the possibility of loss of the usefulness of some ofthe dissolved resin, which would be carried away by the aqueouscomponent or white water which actually liowed through the pulp layerand the porous forming surface. It was also known heretofore that aformed paper sheet or other article might be impregnated with a suitableresin by dipping the paper sheet or other article in a suitable resinsolution, but particularly in the case of relatively thick paper sheets,this often resulted in a failure to impregnate the inner portion of thepaper sheet or other article. This procedure also tended in many casesto impart a glue-like surface on the article, which in many casesresulted in a sticky surface. The resin deposited on the surface of thearticle in this procedure would often tend to block further penetrationof the inner portion of the article. These various diliiculties in theprior art were greatly magniiied, when it was desired to introduce arelatively high concentration of resin in the paper article. This wasparticularly so in the case of attempting to impregnate the paperarticle by dipping the same in an organic solvent of the resin, becausesuch solutions having relatively high concentrations of resins oftenbecame too viscous to penetrate the article satisfactorily.

The instant invention relates to an improved method of incorporatingresins in molded pulp products. The

instant invention also provides an improved method of encapsulatingvarious types of all of the fibers in a molded pulp product with asuitable synthetic resin to provide protection for the individual fiberswhile at the sameV time effecting a measure of improvement in thestrength properties of the molded pulp product.

it is, therefore, an important object of the instant in- Other andfurther aspects, objects and advantages of the present invention willbecome apparent from the detailed discussion thereof as set forthhereinafter, and the drawing attached hereto which shows aself-explanatory flow sheet for the instant method.

In general, the instant invention consists in a method of producingmolded pulp articles, which comprises dispersing finely dividedparticles or fibers, i.e. resin linters of a water insoluble syntheticresin in dilute aqueous stock containing cellulosic iiber, forcing suchstock through a porous forming surface to deposit a layer of mixedparticles and ibers on the forming surface, etecting removal of Waterfrom said layer to substantially dry the same, next contacting saidlayer with a solvent for said particles to effect in situ solubilizationof the resin particles and deposition of the resin in film form ontosaid fibers, and then removing substantially all of the solvent fromsaid layer.

An initial step in the practice of the instant invention involves theprovision of or the preparation of dilute aqueous stock containingcellulosic fibers, with finely divided particles of a water insolublesynthetic resin dispersed therein. The fluid stock employed ispreferably composed of water with bers in suspension therein in arelatively dilute consistency of 0.01% to 2%, and preferably aconsistency of 0.05% on a dry liber basis. (As used herein, the termpercent means percent by Weight unless otherwise specified.) The fibersin the stock are essentially cellulosic fibers, but they may containcertain conventional additives such as clay, pigments, proteins, etc.

The cellulosic bers may consist entirely of the conventional wood pulpfibers as in the case, for example, of kraft pulp, or the dilute aqueousstock may contain a mixture of conventional wood fibers and othercellulosic" iibers such as cotton linters. The cotton linters used inthe practice of the instant invention may be in their natural celluloseform, or they may be modified by partial or complete etheriiication oresteriiication, in accordance with conventional procedures. Such cottonlinters may thus comprise in whole or in part cellulose ethers such asmethyl cellulose, ethyl cellulose, etc. or cellulose esters such ascellulose acetate, cellulose nitrate, etc. In

a preferred process of the instant invention the cellulosic` fibers maycomprise 25% to 75% wood iibers and 25% to cotton linters (particularlygood results being obtained using approximately 40% kraft fibers andapproximately 60% cotton linters). Such stock compositions have beenfound particularly useful in the formation of cylindrical molded pulpproducts having a relatively high cylindrical length-to-diameter ratio.

The synthetic resin is dispersed inthe dilute aqueous stock in the formof finely divided particles by any of a number of conventional methods,which include merely dispersing the resin in powdered form in the stockor initially forming a water base paste, which may be ball milled toeffect a desired initial suspension in the paste which, in turn, isdispersed in the dilute aqueous stock.

An important feature of the instant invention resides in the concept ofusing a water insoluble (or substantially Water insoluble) syntheticresin for this purpose. The resin is reduced to a line particle Sile inpowder or-dispersion paste form, and it is dispersed throughout thedilute aqueous stock in concentrations which may range from as little asabout 1% to as'much as about 100% o'n a dry fiber basis. In the practiceofthe instant invention it is possible to incorporate relatively highproportions of the synthetic resin particles in the molded pulp articlesand preferably the resin concentration is equivalentrto from about 5 toabout 25% on a dry iiber basis. Alternatively, the resin in the form offine fibers, as from spinnerettes, may be blended with the aqueouscellulosic liber stock.

Itis generally understood that the cellulosic fibers in paper stock arenegatively charged, and in the practice of the instantinvention it hasbeen found advantageous to use resins which also carry a negative chargein the dilute basis) are used. In the practice of the instant invention,

however, the incorporation ofthe finely divided particles of the waterinsolublesynthetic resin, even at such higher consistencies, tends toprevent liocculation of the fibers. This is particularly important forthe reason that the flocculatiou of the `fibers at such higherconsistencies characteristically results in poor formation of heavierthick sheets or moldedrpulp products.

YNext, the stock (With the resin dispersed therein) is conveyed Ato andforced through a porous forming surface to deposit a layer of Vmixedresin particles and cellulosic fibers onrthe forming surface. This stepinvolves generallyfconventional web or paper formation Aon a wire' or ona suitable porous mold Wall or surface.V As previously indicated, themolding surfacemay be a forming wire for sheet formation, but preferablyit is a mold for the formation of generally cylindrical articles havingrelatively thick Walls lin the ultimately formed molded pulp article.4The mold Wall may be formed of a material such as individual glassbeads coated with anV epoxy resin to bind the .beads together into themold shape'.v A description of a preferred form of mold material and themethod ofmaking `such material is provided in copend-Y ing patentapplication `of Modersohn and Hornbostel, entitled Moldf U.S. Serial No.89,451, led February 15, The individual resin particles, beingsubstantially insoluble in water, Vare not carried through the porousforming surface and, instead,.the'iibers forming on the mold or Wiresurface tend to trap even the finest particle Y sizes of-resin so thatthe resulting molded pulpproduct consistsessentiallyof a `layerof mixedparticles and fibersV on-the forming surface, WithV the particlesthoroughly 4 involves Washing the pulp layer Yon the forming surfacewith a substantially anhydrous Water-miscible solvent Yin which theresin is substantially insoluble. In the case of the preferred use ofpolyvinyl-aldol resins, a substantially anhydrous alcohol such asmethanol, ethanol, ethylene glycol, etc., is preferably used. Suchalcohois have a comparatively high attraction for water and cannotordinarily be obtained by economically practical processes in a total'-ly anhydrous state. Substantially vanhydrous methanol orY ethanol,however, which may contain onlyS to water, is still capable of taking onsubstantial quantitiesk of water and such an alcoholic system acts as ananhymixed or incorporatedvthroughout the entire body of this pulp layer.v Y

Y After the Vpulp layer has been formed in the desired thickness on theporous forming surface,V it is removed from Contact With the diluteaqueous stock, but the resulting pulp product at this stage stillcontains a substantial Vamount of Water. It will be appreciated thatthepreferred liuid stock'for usefin the practice of the instant inventionis; an laqueous suspension of thercellulosic bers on the resinparticles, but the use ofjother than an aqueous vehicle is withinA thebroad concept ofthe invention;V The aqueous stock system is preferredfor use in the practice Vof the instant invention for obvious economicreasons;

butin certain special cases an alcoholic or'aqu'eousgalcoe holicvehicle, forfexample, might beused'in the practice 'of i the instantinvention, so long as the cellulosic fibers and 1 the resin particlesare both substantially insoluble in such vehicle.Y YIn any event, oncethe molded pulp article has been lformed with the desired VVweb or paperthickness, and

Ythe molded pulp article has been separated from VYthe stock, Y

thestock vehicle retained `in the molded pulp article ispreferablyremovedto a substantial extent. In the case ofaqueousfvehicle, it is desirable vto eliect removal of vi/'afterV fromthe pulp layer VVon the forming surface to Y. substantially-'Vdryrthelayer; e Such' drying or vehicle removaljmay be accomplishedY by`any ofa numberfofconventionalmethods, Ybut particularly `in'thecase oftheformation'of'generallycylindrical molded pulprarticles in a devicehaving the ,desiredmoldshapea it is'desirable to effect the removal ofWater V(or other stock vehicle) while the molded pulp'article isretained'on the porous forming f In such situations, conventional dryingmeans surface.

Y' ,Y arefnot ordinarily preferred; andthe preferred process the Wetsheet,.and 91% fdenatured 'ethyl alcohol Wasj 'i drous or substantiallyanhydrous system, in that it maybe used to Wash the pulp layer(ordinarily using the alcohol in a total amount ranging from about 5 to2,0 times the weight of the layer itself) to effect substantial removalof the Water from the layer. AV nominal amount of Water is retained inthe layer, and this has been found to be useful in the ultimate bondingof the resin to the bers.

After substantially drying the pulp layerY (or removing substantiallyall of the vehicle), the resulting layer is still in the formof mixedresin particles and cellulosic fibers. This layer is then contacted witha solvent for the resin particles to effect in situ solubilization ofthe resin particles and deposition* of the resin in iilrn form onto thefibers.V Preferably, this'is done by washing the pulp layer or moldedpulp Varticle while Vit is still retained on the porous forming surfacewith a suitable solvent. The 'cellulosic fibers are, of course,substantially insoluble in this solvent, butthe resin particles aresoluble therein.: In the case of the preferred polyvinyl-aldol resins,the. solvent used may be ajmixture.y of toluene and ethanol (c g. ablend'of 30-60% toluene and 40-70% ethanol).

lt'will be appreciated thatV excessive Vwashing of the pulp layerl atthis time with such aV solvent might tend to leach a out an appreciablequantity of the resin, but washing ofV the pulp layer with a nominalamount of such solvent to carry out an initial air drying procedure inorder to Y elect suliicient integration of the molded pulp sheet orarticle to permit its removal from Vthe porous forming i surface,followedV by a more extensive, air drying procedu're to removesubstantially all of the solvent.Y Heat mayY also be employed,particularly oncerthe pulp layer has been removed from the porousforming surface, in

order to complete the'rernoval of the solventrand to as?V sist in theformation of a bond between the'bers via the Y resin films. y I Y y.

The invention mayY be demonstrated as follows:

Y Demonstration A Y VTAPlPl hand sheets are formed accordingtofspecilica- Y tion vT205M-58. The pulp usedis bleached kraft Vmadeupto 1% consistency in a beater'without refining.; An f .amount ofpolyvinyl-formal resin in the form of a free owing Vpovvder (Formvar7/95S)f 'equivalent to 25% of the fiber contentof'the Astock (on va dry.liber basis) was dispersed in the stock in Athebeater. After theneyvlyformed WietvsheetV Was made accordingto the''specifica-V tion, va pieceof Fourdrinier'Wir-e was placed on topfof flooded over the wire in aquantity equal to about five times theweight of the wet sheet., Vacuumwas applied i Y Yto thehand sheet to'draw the'sol'vent through jthesheet. 1

Next, the vvire was flooded with about tive times. Y the Weight of theresulting sheet Vof 'anorganic solvent @for the resin'consistingV of 30%toluene .and '770% ethanol;

' and after lthis solvent had Acompletely impregnated'thefa-t, 'j'

sheet, it was removedin part/bythe application of vac? A number ofvariations are possible in the procedure of Demonstration A. Forexample, if the procedure of Demonstration A is repeated except that theber consistency employed in the initial stock is 2%, there wouldordinarily be a tendency to obtain fiocculation in the stock, but itwill be noted that the incorporation of the resin particles in thebeater tends to minimize occulation.

Also, the amount of resin employed may be reduced to 10% and 5%,respectively, of the ber on a dry ber basis, with successive decreasesin the ultimate strength (c g. the tensile strength factor) in the iinalsheet, but even using 5% resin it will be noted that a substantialincrease in strength is obtained in the practice of the invention.

It will also be noted that comparable results are obtained using anorganic solvent in the last step which consists essentially of 40%ethanol and 60% toluene.

Demonstration C A procedure is carried out that is the same as that ofDemonstration A, except that 75% of the kraft bers are replaced withcotton linters, and it is found that substant'ally the same results areobtained. In this procedure the cotton linters used are natural cottonbers (Le. cellulose), but comparable results are obtained in thisprocedure using in place of the natural cotton bers ethered cellulosebers (ie. composed of methyl cellulose or ethyl cellulose) and/oresteried cellulose (i.e. cellulose acetate and/or nitrated cellulosebers).

Demonstration D Stock is processed in a beater to obtain 0.05% berconsistency kraft bers and 75% cotton linters). An amount of theaforementioned polyvinyl-formal resin equal to 20% of the ber content ofthe stock (on a dry ber basis) was formulated into an aqueous paste in aball mill and then added to the stock in the beater to disperse theresin particles throughout the stock uniformly. The stock is then forcedthrough a generally cylindrical porous female mold formed of glass fiberbeads bound together by an epoxy resin (in the manner hereinbeforedescribed) in order to form on the cylindrical forming surface of themold a generally cylindrical Wet molded pulp article. A porous male moldis -then pressed down on the pulp article on the female mold, and 91%ethanol in an amount equal to iive to ten times the Weight of the moldedpulp article is passed through the molded pulp article md both the maleand female molds to substantially dry the molded pulp article. Next, anorganic solvent consisting essentially of 40% ethanol and 60% toluene,in an amount equal to about 5 to 10% of the Weight of the dry moldedpulp article is ushed through the article (and through the male andfemale molds), to effect in situ solubilization of the resin particlesand deposition of the resin in film form onto the bers. Warm dry air isthen forced through the molded pulp article (and the male and femalemolds) to effect at least partial removal of the solvent, and the moldmembers are then separated and the molded pulp article removed therefromand placed in an oven for further heating and drying to complete theremoval of the solvent. The resulting product is found to have a resinlilm substantially encapsulat- 6 ing the bers therein and elfectingincreased strength in the molded article.

Again, the cotton linters employed in the demonstration of the previousparagraph are natural cotton bers, but a comparable improvement isobtained if these cotton linters are esteried (eg. to form celluloseacetate or cellulose nitrate), and it is found that the esteriedcellulose bers are protected and encapsulated by the resin lm so as toobtain a particularly desirable result.

Demonstration E A procedure is carried out that is the same as thatdescribed in Demonstration D except that the resin particles used areformed of polyvinyl acetate resin, and the results obtained arecomparable to those described in Demonstration D.

Demonstration F A procedure is carried out that is the same as that ofDemonstration D, except that the resin particles used are in the form ofa finely ground co-condensate of toluenesulfonamide, melamine andformaldehyde (eg. as described in Example 6 of U.S. Patent No.2,809,954), and the organic solvent used for the particles to elfect insitu solubilization is acetone instead of the alcoholictoluene solvent,and it is found that comparable results are obtained.

In the pratcice of the instant invention, the resin particles used arepreferably thermoplastic resins which are at least reasonably readilysoluble in an organic solvent (eg. alcoholic-toluene, acetone, etc.) foruse in elfecting in situ solubilization in accordance with the end stepof the process. Also, the resins used in the practice of the instantinvention should be substantially insoluble in any solvent that is usedin the removal of the stock vehicle, i.e., the substantially anhydrousalcohol that is preferably used in the removal of water. In addition,the resins used in the practice of the instant invention aresubstantially insoluble in the stock vehicle (Le. Water). The resinswhich may be used in the practice of the instant invention include theWater insoluble acrylic resins (such as the polyacrylamides and thepolyacrylic esters such as polymethyl methacrylate, polyethyl acrylate,etc.), the various Water insoluble polyvinyl resins, and other Waterinsoluble thermoplastic resins such as thetoluenesulfonamide-formaldehyde resins, with or without modication bycocondensation with comparatively small amounts of polyamides such asurea, melamine, etc. Preferably, however, the resins used in thepractice of the instant invention are of the polyvinyl type and the bestresults are obtained using a polyvinyl-aldol resin. The most common ofthese resins are polyvinyl formal, polyvinyl acetal and polyvinylbutyral; and the best results are obtained using polyvinyl-formal. Otherpolyvinyl resins which may be used in the practice of the inventioninclude polyvinyl acetate, polyvinyl chloride-acetate, etc. sincepolyvinyl alcohol is Water-soluble, it is not used as such, but whenmodied by reaction with an aldehyde to form a polyvinyl-aldol such aspolyvinyl-formal, the resin may be used.

Demonstration G Water insoluble powdered phenol formaldehyde resins iare also available for use in the practice of the instant invention (ie.advanced B-stage phenolic resins). The procedure of Demonstration A isrepeated, except that the synthetic resin used is powdered Waterinsoluble phenol formaldehyde resin, and the iinal rinsing or Washingstep With a solvent for the phenolic resin is carried out using anamount of dimethyl formamide that is equal to about three times theWeight of the dried paper sheet, and it is found after removal of thedimethyl formamide that a signicant improvement in strength is obtainedin the resulting paper sheet.

It will be understood that modifications and variations may be elfectedwithout departing fromV the spirit and scope of the novel concepts ofthe present invention.

`I claim as my invention: Y

l. A method of producing molded pulp articles, which comprisesdispel-sing finely divided particles of a water insolublersyntheticresin in dilute aqueous stock containing cellulosic ibers, forcing suchstock through a porous forming surface to deposit a layer of mixedparticles and fibers on the forming surface,'effecting removal of Waterfrom said layer to substantially dry the same by Washing said layer'witha substantially anhydrous Water-miscible solvent in which the VVresin issubstantially'insoluble, nextV contacting said layer with a solvent forsaid particles to eect in situ solubilization of the resin particles anddeposition of the resinin film form onto said fibers, and then removingsubstantially allV of the solvent from said layer,y the amount of,saidssubstantially anhydrous solvent used in the aforesaidwashing stepbeing substantially Vto times the Weight of the layer.

n 2. VA method of producing molded pulp articles, Vwhich comprisesdispersing finely divided particles of a water insoluble synthetic resinin ,dilute aqueous stock containing cellulosic fibers having a berconsistency of 0.0l% to V2% and using resin in a vconcentrationequivalent to from 1% to 100% on dry fiber basis, forcing'such stockthrough a porous forming surface to deposit a layer of mixed particlesand fibers on the forming surface, effecting removal of water from saidlayer to substantially dry the same by Washingrsaid layer with asubstantially anhydrous Y water-miscible solvent in which theresin issubstantiallyV insoluble, nextcontacting said layer With a solvent forsaid particles to effect in situ solubilization of the resin particlesand deposition ofthe resinV in iilm form onto said Viibers, andthenremoving substantially all of the solvent from saidlayer, the amountof said substantially anhydrous solvent used in the aforesaid washingYstep being substantially 5V to 20 times the weight of the layer.

3. A method of producing moldedv pulp articles, which comprisesdispersing finely divided Vparticles ofV a water insoluble polyvinylresin in dilute aqueous stock y.containing cellulosic fibers, forcingsuch stock through a porous forming surface to deposit a layer of mixedVparticles and fibers onthe forming surface, effecting removal of'waterlfrom said layerto substantially dry the same by Washing said Vlayerwith 5 vto A20 times its Weight of a substantially anhydrousWater-'miscible solvent in which the resin is substantially insolube,next contacting said layer with a solvent for saidparticles to effect insitu solubilization of Vthe,resirnrparticles and deposition of the resinin film tacting said layer with a solvent for said particles to effectr`in situ solubilizaton of the resin particles and deposition rof theresinin iilm forrnV onto said fibers, and then reof mixed particles andfibers on the forming surface,

effecting removal of Water from said layer to substantially dry thesaine by passing substantially anhydrous ethanolV through said layer inamounts Vof substantially 5v to V2,() times the Weight of the layer,next contacting said layer with a solvent for said particles to effectin situ solubilization of the resin particles ,and deposition of theresin inV film form onto said fibers, and thenremoving substantially allof the solvent from said layer. Y

6. A method of producing molded pulp articles, which comprisesdispersing finely divided particles of a Water insoluble synthetic resininV diluteaqueous stock containing cellulosic fibers comprising 25% to75% Wood fibers and 25% to 75 cotton linters, forcing suchstock througha porous forming surface to deposit a layer of mixedV particles andfibers on the forming surface, effecting rernoval of water from saidlayer to substantially dry the same by Washing said layer with 5 to 20times its Weight 'of a substantially anhydrous water-miscibleysolvent'in which the resin is substantially insoluble, next contactingsaid layer with a solvent for said particles to effect in situysolubilization of the resin particles and deposition of the i resin iniilm form onto Vsaid fibers, and then removing form Vonto said fibers,and then removing'substantially all of the solvent from said layer. i, Y

f4.' A method of producing molded pulp articles, which comprisesdispersing iinely'divided particles of a water insolublepolyvinyl-aldolresinV in dilute aqueous stock ,containing cellulosic fibers havingnafiber consistency of VY0,01% to 2% and using resin `in a concentrationequiva-k lent to from 1% to y100% .on dry fiber basis, forcing'such ,Ystock' through a porous forming surface to deposit a Y layer of mixedparticles and fibers on the formingsurface,

effecting removal of water from said layer to substantially K dry thesame byV washing said layer with 5 to 20times its Y, V Weight of asubstantiallyranhydrous ivater-miscible solvent inY ivhichthe'resin isrsubstantially insoluble,V nextrconsubstantially all of the Vsolvent fromsaid'layer.

7. A method of producing molded pulp articles, which comprises providinga dilute fluid stock containing' cellulosic fibers and finely dividedsynthetic resin particles in a vehicle in which both'the particles andthe fibers are s insoluble, forcing such stock through a porous formingY surface to deposit a layer of mixed particles and fibers`v on theforming surface, Veecting removal ofthe vehicleYV from said layer bywashing such layer with substantially 5 to 20 times the layer weight ofa solvent that is initially substantially free of the vehiclebut'ismiscible with theY vehicle and in which theparticles and'iibers ofsuch layer are Vsubstantially insoluble, next contacting said layer`with aV solvent for said particles to effect in situ solubilization fof the resin particles and depositionof the resin in film form onto saidfibers, and then removing substantially al of the solvent from saidlayer.

References Citedin the le of'this patent UNITED STATES PATENTS 1,829,585Y Dreyfus et a1. oct. 27, i931 1,848,732 *Y Lionne Marl 8,1932 Y Y YFOREIGN PATENTS i Y 1 565,980 Great Britain nec. 7, 1944 1,083,697France' v .r. r lune 30,'1954

1. A METHOD OF PRODUCING MOLDED PULP ARTICLES, WHICH COMPRISESDISPERSING FINELY DIVIDED PARTICLES OF A WATER INSOLUBLE SYNTHETIC RESININ DILUTE AQUEOUS STOCK CONTAINING CELLULOSIC FIBERS, FORCING SUCH STOCKTHROUGH A POROUS FORMING SURFACE TO DEPOSIT A LAYER OF MIXED PARTICLESAND FIBERS ON THE FORMING SURFACE, EFFECTING REMOVAL OF WATER FROM SAIDLAYER TO SUBSTANTIALLY DRY THE SAME BY WASHING SAID LAYER WITH ASUBSTANTIALLY ANHYDROUS WATER-MISCIBLE SOLVENT IN WHICH THE RESIN ISSUBSTANTIALLY INSOLUBLE, NEXT CONTACTING SAID LAYER WITH A SOLVENT FORSAID PARTICLES TO EFFECT IN SITU SOLUBILIZATION OF THE RESIN PARTICLESAND DEPOSITION OF THE RESIN IN FILM FORM ONTO SAID FIBERS, AND THENREMOVING SUBSTANTIALLY ALL OF THE SOLVENT FROM SAID LAYER, THE AMOUNT OFSAID SUBSTANTIALLY ANHYDROUS SOLVENT USED IN THE AFORESAID WASHING STEPBEING SUBSTANTIALLY 5 TO 20 TIMES THE WEIGHT OF THE LAYER.